yes, you will see a marked improvement immediately with the t-mobile router in use.

no, you could optimize the settings on your non-tmobile router to match the settings and performance of the t-mobile router... and if you have already done this, you wont see a difference except for a logo on your router.

depending on which WRT54 you have... the G, GS, or GL, you could make it even more effective by flashing it with dd-wrt firmware to give you more settings to tweak for better performance. the WRT54GL is the superior router of the bunch for firmware hacking.